Vinyl acetate monomer (VAM) can be prepared in a continuous process with recirculation of the purified product stream (gas recycle process). Here, ethylene reacts with acetic acid and oxygen in a heterogeneously catalyzed gas-phase reaction over catalysts which generally contain palladium salts and alkali metal salts on a support material and may additionally be doped with gold or rhodium. Preference is given to using a Pd/Au catalyst mixture with a potassium acetate promoter.
The starting materials ethylene, oxygen and acetic acid are converted into vinyl acetate in an exothermic reaction (VAM: ΔBH°299=−176 kJ/mol), generally at a gauge pressure of from 7 to 15 bar and, depending on the time on stream of the catalyst, at a temperature of generally from 130° C. to 200° C., in a fixed-bed shell-and-tube reactor or else in fluidized-bed reactors:C2H4+CH3COOH+½O2→CH3COOCH=CH2+H2O
The main secondary reaction here is the total oxidation of ethylene to CO2:C2H4+3O2→2CO2+2H2O
The ethylene conversion is generally from about 5% to 20%, the acetic acid conversion is from 20% to 60% and the oxygen conversion is up to 90%.
Owing to the incomplete conversion of ethylene and the secondary reactions, a gas mixture consisting predominantly of ethylene, carbon dioxide, ethane, nitrogen and oxygen (recycle gas) is circulated in the preparation of vinyl acetate. The recycle gas is admixed upstream of the fixed-bed shell-and-tube reactor with the starting materials acetic acid, ethylene and oxygen and brought to the reaction temperature by means of heat exchangers operated using hot steam. The enrichment of the recycle gas with acetic acid is usually effected by means of an acetic acid saturator heated by means of hot steam.
After the reaction, the reaction products vinyl acetate and water and unreacted acetic acid are condensed out from the recycle gas, preferably in a predewatering column, and passed to the further work-up. Starting material/product which has not been condensed out, essentially ethylene, CO2 and vinyl acetate, can be taken off at the top of the predewatering column and the vinyl acetate can be scrubbed out in a scrubber operated using acetic acid (recycle gas scrubber). The overhead product from the predewatering column which has been treated in this way, namely the recycle gas, or at least part thereof, can then be freed of carbon dioxide formed in a CO2 scrubber. The recycle gas is optionally compressed, replenished again with the starting materials and introduced into the reactor for gas-phase oxidation.
For the further work-up of the bottom product from the predewatering column and the bottom product from the recycle gas scrub, the product vinyl acetate and water and also unreacted acetic acid which have been condensed out can be separated from one another in a multistage distillation process which is usually operated using hot steam. The customary distillation steps for isolating the vinyl acetate and the acetic acid are azeotrope column, dewatering column, pure VAM column and also columns for residue work-up and removal of low boilers and high boilers.
DE-B 1278430 describes a process in which the product mixture leaving the reactor is fractionated in a distillation column filled with saddle bodies. At the bottom of the column, an acetic acid/water mixture which is free of vinyl acetate is taken off. The distillate obtained at the top of the column is condensed and separated into an aqueous phase and an organic phase, with the organic phase consisting of vinyl acetate, water, acetic acid and acetaldehyde being recirculated into the distillation column. A disadvantage of this process is the relatively high proportion of acetic acid in the overhead product from the column, which makes the phase separation of vinyl acetate and water difficult and therefore makes an additional separation by distillation of vinyl acetate and acetic acid and also of acetic acid and water necessary.
The process of DE-C 2610624 concerns the predewatering of the reaction mixture formed in the vinyl acetate synthesis. In this process, the product mixture is introduced into a predewatering column. A mixture of vinyl acetate, water and acetic acid is taken off from the bottom of the column. The gaseous overhead product is condensed and, after phase separation of the condensate, the organic phase is, after having been heated in a heat exchanger, recirculated into the predewatering column and the aqueous phase is passed to wastewater treatment. A disadvantage of this process is the high temperature at which the gaseous reaction mixture is transferred into the predewatering column. Measurements have shown that at relatively high entry temperatures the column surprisingly has higher ethyl acetate values at the top of the column, which makes the removal of ethyl acetate difficult. The ethyl acetate then has to be distilled off from the bottoms from the recycle gas scrubber, which is costly. In the process of DE-C 3422575 A1, too, the gas mixture leaving the reactor is, as in DE-C 2610624, introduced without prior cooling into a predewatering column, with the disadvantages just described.
EP 0423658 A2 describes a process in which the product gas mixture leaving the reactor is fed with or without cooling into the predewatering column. The process is concerned in particular with the fractional distillation of the bottom product from the recycle gas scrubber in a column which is additional compared to DE 3422575 A1. The advantage of the process is that a smaller number of column plates is needed in the work-up by distillation.
In EP 1760065 A1, the mixture leaving the reactor is introduced into a predewatering column which is constructed as tray column. A description is given of a process in which part of the bottom product from the recycle gas scrubber is recirculated to the predewatering column.
In the process described in WO 2014/036132 A1, the product gas stream from the reactor is fed into a predewatering column. A product gas stream whose composition differs from that of the product gas stream fed in is taken off from the column and is fed to a second reactor for the gas-phase oxidation. Both tray columns and packed columns are recommended as predewatering column.